1. Field of the Invention
This invention relates to an image information signal transmitting system for transmitting an image information signal.
2. Description of the Related Art
In transmitting information such as image information, it has been considered important to find a way how to reduce the transmitting amount of information while ensuring reproduction of the original image with fidelity. Hence, various transmission methods have been proposed. They include an adaptive type variable-density sampling method by which sampling density, that is, the density of transmitted information is suitably changed. This method is based on the following concept: Generally, areas of a high spatial frequency and areas of a low spatial frequency commingle within an image. The transmitting amount of information can be adequately reduced by increasing the sampling density for the area of a high spatial frequency and by decreasing it for the area of a low spatial frequency. In accordance with the adaptive type variable-density sampling method, a picture is divided into areas of high sampling density and areas of low sampling density on the basis of the spatial frequency or the like for the purpose of keeping the transmitting amount unvarying for each picture as a whole. However, according to this method, some area that has a relatively low spatial frequency tends to be erroneously allotted to the low density sampling. Although the error may not be serious for a moving image part of the picture, it shows up as deterioration of picture quality in a still image part for which the resolution of the human sight becomes high.
Meanwhile, there has been proposed a transmission system which is arranged to enhance picture quality by utilizing the fact that such continuous images as those of an ordinary TV signal are correlative with each other even in the direction of time base. In other words, the transmission system takes into consideration the correlativity in the time base direction in addition to the two-dimensionally expanding image information. The signal receiving side of the system does not require renewal of picture element data and permits omission of signal transmission at least for the still image area of the picture. This omission permits the transmission density to be increased for other areas to enhance picture quality. Hereinafter the method of utilizing the correlativity in the time base direction will be called the three dimensional TAT (time-axis transformation) method.
In the case of the three-dimensional TAT method, one picture is divided into a plurality of picture element blocks. The amount of information to be transmitted for one picture is arranged to be unvarying. For example, each picture element block is controlled in a prescribed manner in such a way as to have the transmitting amount of information for one picture compressed at a rate of 1/2. The one of the following modes is allotted to each of the picture element blocks: A mode in which data for all the picture elements included in the block is transmitted (hereinafter referred to as a mode "e"); another mode in which data only for basic picture elements of the block is transmitted (hereinafter referred to as a mode "c"); and a further mode in which data for the corresponding block of a previously transmitted picture is utilized (hereinafter referred to as a mode "p"). With one of these different modes allotted, the image is reproduced with a signal processing operation corresponding to the allotted mode carried out on the signal receiving side. A signal indicative of allotment of the transmission mode is separately transmitted as a mode information signal. Further, the mode "p" may be arranged in the following manner: The basic picture element data is first transmitted alone like in the case of the mode "c"; then, the data transmitted is compared with the data of the corresponding block of the preceding picture on the signal receiving side; and, if the currently received data is found to be the same as the previously received data, the transmission mode is determined to be the mode "p" instead of the mode "c". The details of the conventional three-dimensional TAT method are as described below:
In the conventional three-dimensional TAT method, a thinning-out and interpolating operation is first carried out. After that, a block distortion Dc taking place during transmission in the mode "c" is computed beforehand by comparing the result of the thinning-out and interpolating operation with a real value. Further, for finding the correlativity in the time base direction, the image signal of the preceding picture is stored in a frame memory. Another block distortion Dp which represents timewise correlativity is computed by comparing the picture element data of the preceding picture with that of the current picture for every picture element block. The block distortion values Dc and Dp are compared with each other. As a result, a discrimination signal Dc/Dp indicating which of the two values is larger is obtained. The smaller block distortion is used as transmission mode determining block distortion Dm. In other words, a check is made for every picture element block to find which of the mode "c" transmission and the mode "p" transmission has a less block distortion. If block distortion Dc of the former is found larger than the block distortion Dp of the latter, the mode "c" is not selected. If latter distortion Dp is found to be larger than the former distortion Dc, the mode "p" is not selected. However, a picture element block which is transmitted in the mode "c" for the preceding picture cannot be improved in picture quality if it is transmitted in the mode "p" for the current picture. In view of this, a picture element block transmitted in the mode "e" for the preceding picture is transmitted in the mode "p" for the current picture.
The block distortion is thus computed for all the picture element blocks constituting one picture. After that, the transmission modes are allotted to the picture element blocks as applicable in such a way as to have the transmitting amount of information for one picture unvarying from a given value which is set, for example, at 1/2 in terms of a compression rate. Then, the picture element data of each picture element block is transmitted one after another in the mode allotted. On the signal receiving side, each of the transmitted data is compared with the basic picture element data of the same block received for the preceding picture. If the data is found to be the same as that of the preceding picture, the transmission mode is determined to be the mode "p" and the data of the preceding picture is utilized for image reproduction. If the data is found to differ, the mode is determined to be the mode "c" and the image is reproduced through an interpolation process.
FIG. 1 of the accompanying drawings shows the mode distribution for the block distortions Dc and Dp. FIG. 2 shows the ratio of distribution determined according to the timewise correlation of images. A block having a greater degree of movement is located in a higher place on the axis of Dp. Meanwhile, a block having a higher degree of fineness, i.e., a block of a two-dimensionally higher frequency is located farther rightward on the axis of Dc. The mode "c" or the mode "p" is selected according to the magnitude relation between the block distortion values Dc and Dp. Therefore, the picture element blocks located above the straight line Dc=Dp of FIG. 1 are transmitted basically in the mode "c" and the blocks located below the straight line in the mode "p". The value Dm of a picture element block located at a point Xc is a value obtained on the axis Dc by drawing a perpendicular line. The value Dm of a picture element block located at a point Xp is a value obtained on the axis Dc by drawing a first perpendicular line to the axis Dp and further by drawing a second perpendicular line to the axis Dc from an intersection between the first line and the above-stated straight line Dc=Dp. On the axis of Dm of FIG. 1, there is shown a threshold value T1 which is to be used in selecting the mode "e". This value T1 becomes a threshold value T2 on the axes Dc and Dp. In other words, this means that any picture element block having a vigorous movement and a high degree of fineness is transmitted in the mode "e".
The mode allotment distribution ratio is as follows: Assuming that the data compressing rate for one whole picture is fixedly set at 1/2 and that the amount of picture element data to be transmitted in the modes "c" and "p" is 1/4 of the amount of the picture element data constituting one picture element block, the number of picture element blocks to be transmitted in the mode "e" is 1/3 of the whole picture. In other words, as shown in FIG. 2, 1/3 of the total number of blocks are transmitted in the mode "e". The rest are transmitted either in the mode "p" or in the mode "c" according to the block distortion Dc or Dp. In FIG. 2, the part on the right side of a straight line corresponds to a case where there is no correlation between the preceding picture and the current picture. A part on the left side of the straight line corresponds to a case where a completely still image is transmitted. This gives the same degree of resolution as in the case of transmitting all the picture element blocks in the mode "e". The mode allotment distribution rate of any picture is indicated by the lengths of line segments of a broken line A defined by areas e, c and p in FIG. 2. The position of the broken line A varies with the timewise correlativity of the image information.
In order to transmit the image information as efficiently as possible within the limited transmitting amount of image information, the modes "e", "p" and "c" must be allotted in a manner apposite to the characteristic of the visual sensation of the human eye and that of the transmitting system. However, the conventional system is merely arranged to simply compare the block distortions Dc and Dp with each other and to use the smaller of them as block distortion Dm for determining the transmission mode. Therefore, even in the case of a picture element block wherein the distortion Dp is only slightly larger than the distortion Dc, the mode "c" is allotted to the block. Further, the sampling clock signal has a considerable amount of jitters in terms of hardware. In processing a still image, therefore, the jitters might cause the mode "c" to be allotted by mistake to a picture element block to which the mode "p" should be allotted. Some noise that is included in the signal source might likewise bring about the same problem. Further, even if the block distortion Dp is accurately detectable, the selection between the distortions Dp and Dc simply on the basis of a smaller value does not always lessen the deterioration of picture quality in terms of visual sensation, because the nature of the block distortions Dp differs from that of the block distortion Dc.